Focusing arrangement for electron discharge devices of electron velocity modulation tpe



Oct. 4, 1949. J. H. FREMLIN 2,483,407

' FOCUSING ARRANGEMENT FOR ELECTRON DISCHARGE DEVICES OF ELECTRONVELOCITY MODULATION TYPE Filed Nov. 9, 1946 L/ Ipuentor \77/14 F/FEML wAttorney Patented Oct. 4, 1949 UNITED STAT ES RATE NT' GFFICE- 2,483,407I FooUsiNG ARRANGEMENT. Eon ELECTRQN DISCHARGE DEVICES. or ELECTRON VE-LQCLTY MODULATION TYPE.

.Iohn Heaver Fremlin, Iiondom England, assignor to InternationalStandardi Eleotrie-(torporafiom.

Ne York, Y.

Application November 9, 1946, Serial No. 709,020

'In Great Britain August 31-, 1945* Section 1,, Public Law 69.0,,August8,1946

Patent expires August 31 1965 2 Claims. (01. 250-161) The presentinvention relates to improvements in magnetic focussing arrangements forelectron discharge devices of the electron velocity modulation type.

In order to obtain satisfactory operation of such devices with lowoperating voltages it is necessary to employ a high current density inthe electron beam, and for this purpose it is common practice to focusthe beam magnetically by means of a strong and preferably smallpermanent magnet. Some trouble has been experienced in the past inassuring that the magnetic lines of force (which determine the paths ofthe electrons in the beam) will be in exactly the proper direction withrespect to the device, since manufacturing variations cause differencesin different samples of the same design of magnet and of the electrondischarge device itself which render some adjustment essential.

In the copending U. S. application of S. G. Tomlin, Serial No. 584,903,filed March 26, 1945, there is described one method of dealing with thisdifficulty. However, when the electron discharge device is enclosed in acylindrical glass envelope, it is impracticable to manufacture theenvelope to close tolerances in diameter, and so rather large clearancesmust be allowed between the envelope and the poles of the magnet toallow the device to be rotated in order to make the adjustment in themanner described in the abovequoted application. This means that the gapbetween the poles has to be inconveniently large, and the magnet has tobe much larger than is desirable in order to obtain the necessary fieldstrength.

If, however, the envelope of the device be made of metal such as copper,it need no longer be made circular in section, but can be rectangular,and can be manufactured to close limits. This will allow the gap in thefocussing magnet to be considerably reduced, and it need only be madejust large enough to accommodate the envelope. This will, however,prevent any rotation for adjustment of the direction of the magneticfield, and it is therefore the principal object of the present inventionto provide means for making the adjustment when relative rotation of thedevice and the magnet is not possible. According to the invention,therefore, the focussing magnet is divided into two similar portions orhalves which are slidable one with respect to the other in such manneras to vary the direction of the lines of force which pass between thepole pieces.

The invention will be described with reference to the accompanyingdrawing, in which Fig. 1 shows a view of an electron discharge devicetogether with the focussing magnet according to the invention;

Fig. 2 shows a partly sectional view of another form of focussing magnetaccording to the invention; and

Fig. 3 shows a side view of the arrangement of Fig. 2.

Referring to Fig. 1, a section of the envelope of the electron dischargedevice is shown in outline at I, but no internal details are shown. Thefocussing magnet is out along the line 2 into two similar halves 3 and4. The electron beam is intended to be projected across the envelope 1perpendicular to the pole surfaces of the magnet halves, but owing tomanufacturing variations, the direction of the beam may be at a smallangle to the intended direction as indicated, for example, by the arrow5. Assuming that before the magnet was cut, the lines of force would runsymmetrically across the gap, when the halves are displaced in themanner shown, the field will be slightly distorted so that its generaldirection is inclined in the manner indicated by the lines 6. It will beevident that the magnetic field may in this way be made substantiallyparallel to the arrow 5.

It will be evident also, that if before the magnet was cut, the lines offorce were not quite symmetrically placed in the gap, a slight mutualadjustment of the two halves 3 and 4 will align the field with the arrow5 in the case when the latter happens to be perpendicular to the polepieces, so that the adjustment according to the inven tion takes care ofmanufacturing variations both in the magnet and in the electrondischarge dev1ce.

Figs. 2 and 3 show the invention applied to a magnet of somewhatdifferent form. The magnet comprises in this case two similar halves land 8 forming a closed ring embracing the envelope I of the electrondischarge device. These halves will be magnetised so that the north andsouth poles are located in the narrow parts as indicated, so that afield transverse to the envelope l is produced. The two halves l and 8are held together by two screws 9 and I0 fixed in holes of oval orelongated section to allow a certain amount of lateral adjustment of thetwo halves,

as indicated by the arrows. It will be evident that the magnetic fieldmay be tilted either way by this means.

It should be noted that while it is generally preferable to employpermanent magnets for focussing the electron beam, the same principles 3could be applied to electro-magnets. Thus, for example in Fig. 1, theportions 3 and 4 could each be of soft iron, and each could be providedwith a suitable magnetising winding (not shown) What is claimed is:

1. An electron discharge device having a section non-circular incross-section and a focussing magnet having pole pieces closelyembracing said section, said magnet comprising two similar magnetportions in end to end abutment and slidable one over the other on atleast one surface of contact 2. In an electron discharge devicearrangment, a focussing magnet comprising two similar magnet portionsabutting each other on and slidable one over the other on a pair ofoppositely polarized pole surfaces, a second pair of pole surfaces ofopposite sign closely adjoining either side of said device.

JOHN HEAVER. FREMLIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

